Freshwater mussels of the superfamily Unionidea (unionids) are among the most imperiled groups of species on the planet. Human alterations to the rivers in which they live, such as impoundments (dams), are considered one of the major factors in the decline of these organisms. Other authors suggest that adult survivorship is high in at least some southeastern impoundments. This suggests that something else, like disruption of the reproductive process, may be the cause of this decline. Unionid mussels have a larval lifestage called glochidia that are obligate parasites, primarily on the gills and fins of fish. About one in 10,000 glochidia are expected to survive to the juvenile lifestage because of the low probability of finding and parasitizing a host fish. Habitat changes, as caused by impoundment, can affect the survival of host fishes and the glochidia and thus the structure of the unionid populations in general. We examined the relationship between the glochidia of the unionid fauna of the Tombigbee (impounded) and Buttahatchee (free-flowing) rivers and three species of known glochidial host fishes(Campostoma oligolepis, Lepomis megalotis, and Micropterus salmoides) collected before and after impoundment of the Tombigbee. We tabulated the change in the average number of glochidia per parasitized fish and the percentage of fish parasitized. Differences in the average glochidial load on L. megalotis was the sole significant difference detected in preliminary samples, while the proportion of individuals infested did not differ in pre- versus post-impoundment samples of any of the three species.